This invention relates to apparatus and methods in which the Raman effect is used to analyse a sample.
The Raman effect is a phenomenon in which a sample scatters incident light of a given frequency, into a frequency spectrum which has lines caused by interaction of the incident light with the molecules making up the sample. Different molecular species have different characteristic Raman spectra, and so the effect can be used to analyse the molecular species present.
Analysis methods and apparatus using the Raman effect are described in a paper `Raman Microprobe and Microscope with Laser Excitation`, M Delhaye and P Dhamelincourt Journal of Raman Spectroscopy, 3 (1975) 33-43. A sample is irradiated with monochromatic light from a laser, and the scattered light is passed through a monochromator in order to select a particular line of the resulting Raman spectrum. The paper describes both a microprobe, in which light from a single illuminated point or a line on the sample is passed through the monochromator, and a microscope in which an area is illuminated and an integral, two dimensional image of that area is passed through the monochromator. The microprobe has the disadvantage that in order to obtain a two dimensional image, it is necessary to scan a series of points or lines over the area of the sample, so that building up the required image is complex and may take a relatively long time. The microscope obviously does not suffer from this disadvantage,.but the optics of the monochromator require substantial modification in order to pass a two dimensional image.
Specifically, a conventional monochromator has an optical system which focuses an image of the illuminated point or line of the sample onto an entrance slit; and a further optical system which focuses an image of the entrance slit onto an exit slit. Between the entrance slit and the exit slit there is a dispersive device such as a diffraction grating (or commonly two or three such gratings in series). The dispersive device has the effect of splitting an incoming polychromatic light beam into a range of angles, depending on frequency. Because of the dispersion, the position of the exit slit relative to the diffraction grating selects the desired spectral line to be investigated. The monochromator can be tuned to different spectral lines by moving the exit slit, or more conveniently by an arrangement in which the diffraction grating is rotated relative to the exit slit. Because the frequencies of the spectrum are separated by a dispersive process, it is obvious that good frequency resolution requires narrow slits. Since the image of the sample is focused on the slits, this is the reason why this conventional monochromator arrangement cannot observe a two dimensional area of the sample. If a wider slit were used in order to pass a two dimensional image, it would pass a range of frequencies. Because any given spectral line has a finite width, the result is a blurred image of any given point on the sample, and if one attempts to form an image in two dimensions, the blurred image of one point in the sample will overlap with the blurred image of an adjacent point, resulting in a very confused image (poor spatial resolution in addition to degraded frequency resolution).
The modified optical system used in the above paper in order to provide an integral two dimensional image, forms an image of the sample on the diffraction grating of the monochromator, instead of on the entrance and exit slits. At the entrance and exit slits, there are formed images of the exit aperture of an optical microscope which views the area of the sample which is to be imaged. By these means (in theory) one can pass an integral two dimensional image of the area of the sample through a monochromator with arbitrarily narrow entrance and exit slits. However, the aperture size of the entrance slit governs the amount of light which is collected and focused onto the grating; while the aperture size of the exit slit governs the amount of light which is collected from the grating and focused to produce the resulting two dimensional image which is detected. In the result, therefore, if one makes the entrance and exit slits narrow, to improve the frequency resolution, then the intensity of the resulting image is extremely low and difficult to detect. This is exacerbated by the fact that the desired Raman spectra are already of very low intensities, and cannot be increased merely by increasing the incident illumination of the sample by the laser, since increased laser power is likely to destroy the sample. Accordingly, commercial Raman analysis devices have tended to be of the scanning microprobe type, rather than a Raman microscope in which an integral two dimensional image is formed.